1. Field of the Invention
The present invention relates to a radar apparatus having an automatic tuning function.
2. Description of the Related Art
A magnetron has been frequently used as an oscillator for a radar apparatus equipped to boats and ships, etc., however, the oscillation frequency of the magnetron is generally unstable in many cases. When the oscillation frequency of the magnetron varies, the frequency of a reflection signal from a target also varies, and thus an excellent radar picture cannot be achieved.
A radar apparatus having an automatic tuning function as shown in FIG. 7 is known as a radar apparatus which can solve the problem as described above.
FIG. 7 shows a conventional radar apparatus. In FIG. 7, a pulse signal generated in a magnetron 101 is transmitted through a transmission/reception switching unit 102 to an antenna 103 in response to a transmission trigger, and irradiated to a detection area as electromagnetic waves. An echo signal reflected from a target is received by the antenna 103, passed through the transmission/reception switching unit 102, and then combined with a local oscillation signal from a local oscillator 105 to be converted to an IF signal (a signal in an intermediate frequency band) by a mixer 104. A local oscillator whose oscillation frequency is variable by changing a control voltage to be applied is used as the local oscillator 105.
An IF signal output from the mixer 104 is transmitted through an amplifier 106, a band filter 107 and a wave detector 108 to a main bang signal extracting unit 109. The main bang signal means a signal generated when a part of a pulse signal generated in the magnetron 101 leaks to the reception system through the transmission/reception switching unit 102 by reflection or the like without being irradiated from the antenna 103 to the air. In the main bang extracting unit 109, the main bang signal is extracted from the input signal, and input to an envelope peak detector 111. In the envelope peak detector 111, the peak value of the intensity of the input main bang signal is accumulated over the detecting operation of the radar which is executed at plural times while the control voltage applied to the local oscillator 105 is successively changed. In this case, the control voltage of the local oscillator 105 with which the peak value of the intensity of the main bang signal is maximum is determined, and then the determined control voltage is applied to the local oscillator 105 when the subsequent transmission operation is carried out. The signal other than the main bang signal is transmitted as a video signal to a display unit 110, and a detection image is displayed on the display unit 110.
FIGS. 8A and 8B are diagrams showing an aspect (tuning operation) that the control voltage applied to the local oscillator 105 is changed every transmission. Under radar operation, the tuning processing as described above is automatically executed periodically. As shown in FIGS. 8A and 8B, the control voltage applied to the local oscillator 105 is slightly varied upwardly and downwardly (tracking) at a period of two seconds, thereby determining the control voltage under which the peak value of the main bang signal is maximum.
Furthermore, U.S. Pat. No. 5,654,716 (Patent Document 1) discloses a radar apparatus based on a different system from the foregoing device.
According to this radar apparatus, the number of waves of a signal whose frequency is required to be detected is counted for a fixed time, and the thus-achieved count value is compared with the number of waves of a reference signal which is counted for the same time, thereby estimating the value of the frequency.
FIG. 9 schematically shows the radar apparatus disclosed in the Patent Document 1.
As shown in FIG. 9, an IF signal output from a mixer 204 and a reference signal generated by a compensation signal generator 207 are input to a comparator 208 through a multiplexer 206, and the respective signals are shaped into rectangular waves. Thereafter, each of the IF signal and the reference signal converted to the respective rectangular waves is cut out for a fixed time by a gate 209, and the number of the cut-out waves of each signal is counted by a counter 210. In the radar apparatus disclosed in Patent Document 1, the numbers of the respective waves of the IF signal and the reference signal which are counted over the same time period are compared with each other to estimate the frequency of the IF signal, and the voltage applied to the local oscillator 205 is controlled by a tunable voltage generator 211 so that the frequency of the IF signal is coincident with a predetermined target value.
In the radar apparatus shown in FIG. 7, it is required to upwardly and downwardly change(increase and reduce) the voltage applied to the local oscillator 105 when a voltage applied to the local oscillator 105 under which the peak value of the intensity of the main bang signal is maximum is determined. Therefore, when the above processing is executed under the state that the tuning has been already established, the applied voltage is changed so that the frequency is out of tuning, so that the detection image is disturbed (see FIG. 8A). Furthermore, the maximum value of the peak value of the intensity of the main bang signal is not known unless the applied voltage is changed upwardly and downwardly, and thus it is impossible to judge in advance whether tuning has been already established or not.
Furthermore, in the radar apparatus disclosed in Patent Document 1, the IF signal and the reference signal which are converted to the rectangular waves are cut out for a fixed time and compared with each other. Therefore, the high-precision compensation signal generator for generating the reference signal and the high-precision gate 209 for accurately cutting out the rectangular wave for a fixed time are required, however, all these units are expensive. Furthermore, in order to enhance the precision of the detection frequency, the sampling frequency is increased and averaging is required, so that it takes much time to execute the processing and thus it is difficult to follow the quick frequency variation of the magnetron.